• Planète et Univers/Océan, Atmosphère
  
• Planète et Univers/Interfaces continentales, environnement
  
• Sciences de l'environnement/Milieux et Changements globaux
  
• Sciences de l'environnement/Biodiversité et Ecologie
  
• Sciences du Vivant
  

The effect of silica and carbonaceous matrices (charged mineral surfaces) in phytoplankton cells on the transfer of singlet oxygen from irradiated phytodetritus to their attached bacteria was investigated under controlled laboratory conditions. Our results indicate that a silica matrix (i.e. as in diatom frustules) inhibits the transfer of singlet oxygen and limits the induced photodegradation of cis-vaccenic acid (a fatty acid generally considered as specific to bacteria). In contrast, a carbonaceous matrix (i.e. as in coccoliths) does not seem to inhibit the transfer probably due to the release of coccoliths upon cell death. As a consequence, bacteria associated with phytodetritus from diatoms should be in a healthy state and biodegradation of organic matter associated with these particles should be favoured. These results should contribute to a better understanding of photosensitized degradation processes and to a better estimation of the balance between degradation and preservation of organic material during sedimentation in seawater.

Diatoms are a key phytoplanktonic group that affects the carbon cycle in the ocean. Although the effect ofnutrient limitation on the primary productivity of diatoms is well-studied, the effect of such a limitation on theorganic matter quality of diatoms and their vertical transport through the water column remains unclear. Inthis study, the diatomThalassiosira weissflogii (TW)was grown under twodifferent nutrient conditions, ‘N-stress’and ‘Si-stress’, and was compared against healthy TW cells (Nutrient-replete). Biodegradation experiments ofTW were performed for all of the above conditions, and the particulate fraction was monitored over time(~one month) in terms of the organic carbon (POC), nitrogen (PON), and sugars (PCHO), including prokaryoticcounting. Using these results, we estimated the degradation rate constants for POC, PON, PCHO, and theindividual carbohydrate monomers (monosaccharides) for the TW cells. Our results indicated that the N- andSi-limitations increase the organic carbon content of the TW cells with a concomitant decrease in the siliconcontent (bSiO2), suggesting a modification of the TW cells. The PCHO content increased by a factor of 2.6 and3.8 in the N-stress and Si-stress cells, respectively. At the beginning of the experiment (T0), the N-stress andSi-stress cellswere characterized by higheramounts of glucose (5–32mol%) and xylose (13–19mol%), comparedwith the Nutrient-replete cells, which were dominated by ribose (~22 mol%), indicating differences in the physiologicalstatus of the TW cells and/or in the synthesis of storage/structural polysaccharides. The first orderdegradation rate constants (k1) for the POC were similar in all of the experiments (k1 = 0.096–0.113 d−1),which was not the case for the PON in which the highest values (by a factor of ~2.5) were observed for thenutrient-replete experiment. This result indicates a different behavior in the utilization and/or accessibility toprokaryotes of carbon and nitrogen during the biodegradation experiment. Moreover, ribose, glucose, andgalactose exhibited the highest degradation rate constants in all of the experiments, which further reflects thedifferences in their initial macromolecular origin (e.g., storage vs structural carbohydrates) and highlights thechanges in the organic matter quality during growth under nutrient-limited conditions and degradation. Theseresults suggest that the ‘nutrient-stress’ diatoms may affect the export of carbon (particularly carbohydrates)relative to nitrogen in the ocean interior compared with the diatoms grown under optimal conditions.

Diatoms are major actors in the export of organic carbon out of the euphotic zone. Yet, the processes linking biogenic silica and carbon sedimentation fluxes to deep oceanic layers remain unclear. Analysing organic fractions in biominerals is challenging because efficient cleaning often led to structural alteration of organic molecules. Hence, although lipids are widely used as biogeochemical markers in ocean flux study, few studies have dealt with the lipids that are associated with frustules. In the present study, a protocol was set up to extract and quantify the fatty acids associated to the frustule of the diatom species Thalassiosira weissflogii. The protocol involves solvent extraction of diatom external lipids, followed by clean frustule dissolution by 4% NaOH during 1h at 95°C and subsequent solvent re-extraction of frustule-associated lipids. Results confirmed that this protocol was efficient first, to isolate the frustule from the rest of the cellular organic carbon and second to extract and quantify fatty acids (FA) associated to frustules of this species. FA composition of the frustules was significantly different from that of the whole cells consisting primarily of 14:0, 16:0 and 18:0 FA, as well as a smaller portion of 16:1 and 18:1 unsaturated FA. Frustule-associated FA constituted 7% of the total FA and 1.8% of the total POC. The 30 days T. weissflogii degradation/dissolution experiment suggested that frustule FA 14:0 and 16:0 were mainly associated with the bSiO2 phase dissolving slowly as no degradation of this pool was measured despite 78% frustule dissolution. At the end of the degradation experiment, this pool constituted 5.8% of the remaining total POC suggesting an effective protection by the frustule through strong interaction with the biogenic silica which is consistent with the correlation observed at depth between Si and POC sedimentation fluxes.